Pathogen impacts on plant communities

Parasitism is one of the most common strategies of life on earth, and natural plant communities host a huge diversity of pathogens. Our work examines feedbacks between pathogen and host communities in California grasslands and other systems. We are growing fungal pathogens of native and exotic grasses in culture to identify the pathogen community in this vast and heavily invaded ecosystem. In tandem, we are experimentally measuring pathogen impacts on plant competition and other demographic rates. Combined with mathematical models, this work will help us understand how pathogens affect the composition of plant communities. We are particularly interested in how generalist pathogens that can be transmitted between host species may indirectly modify competition. In California grasslands, where native and exotic grasses compete intensely and share pathogens, disease may be a major driver of coexistence and competitive exclusion. This work takes place in the amazing and beautiful Jasper Ridge Biological Preserve, just seven miles from Stanford University main campus.


Effects of temperature on vector-transmitted diseases

Mosquitoes and other biting insects transmit many of the most important, devastating, and neglected human infectious diseases, including malaria, dengue, chikungunya, and West Nile virus. Because these disease vectors are small-bodied and cold-blooded they are sensitive to environmental temperature. As a result, climate change is likely to shift the global distribution of these vector-borne diseases. We have used laboratory data and a mathematical model to show that thermal responses of traits involved in malaria transmission are nonlinear, and that the resulting optimum temperature for malaria transmission is just 25°C. The model allows us to make nuanced predictions about where climate change may most impact malaria transmission. In a new project funded by the National Science Foundation Ecology and Evolution of Infectious Diseases Program, we are working to predict thermal responses in 13 vector-borne diseases, to map these predicted changes, and to estimate their uncertainty using mathematical models and Bayesian statistics. Our work will also explore local thermal adaptation in two key mosquito species that transmit dengue, chikungunya, zika, and yellow fever: Aedes aegypti and Aedes albopictus. Collaborators include Leah Johnson, Sadie Ryan, Krijn Paaijmans, Samraat Pawar, Kevin Lafferty, Jason Rohr, Van Savage, Matt Thomas, Anna Stewart Ibarra, and others.

Mechanistic models that incorporate realistic relationships between the environment, mosquitoes, and parasites are sometimes difficult to apply in the field. In a project funded by the Center for Innovation in Global Health, in collaboration with Desiree LaBeaud, we are working to develop field-ready predictive models for dengue transmission and to test them using field data on mosquitoes and human cases in Kenya. We're also involved a working group on integrating environmental and size variation into vector transmission models, through the Vector Behavior in Transmission Ecology (Vector BiTE) Research Coordination Network, along with Courtney Murdock, Cynthia Lord, Amir Siraj, Tim Keitt, Chris Stone, Zach Batz, Whalen Dillon, Jeremy Cohen, Jason Rohr, and Marta Shocket.


Maintenance of parasite diversity

Parasites live in diverse communities, competing for host resources. What mechanisms allow all these parasites to coexist? We have studied how several life history tradeoffs may promote parasite coexistence. In California salt marshes, we have shown that a competition – colonization tradeoff promotes coexistence for trematode parasites, but that other mechanisms may also contribute. In grasslands, we have shown that competing viruses coexist by coinfecting host plants and via a tradeoff between transmission efficiency and the ability to use multiple vector species. This work is in collaboration with Alejandra Jaramillo, Kevin Lafferty, Ryan Hechinger, Jake Ashford, Charles Mitchell, and Kevin Gross.